Writer-director Justin Zackham has one incredible asset at his disposal for “The Big Wedding”: an exceptional cast, which includes Robert De Niro, Susan Sarandon, Diane Keaton, Amanda Seyfried and Robin Williams.

Sadly, superior talent can propel a movie only so far. Bad scripts beget bad movies, even when four Academy Award winners are involved.

The story, based on the 2006 Swiss-French film “Mon Frere Se Marie,” launches just before the wedding of the Griffin family’s youngest, adopted son. The fraught, joyous event promises to reunite the whole clan. The problem is that patriarch Don (De Niro) hasn’t seen his ex-wife, Ellie (Keaton), in 10 years, and he’s currently shacked up with her best friend, Bebe (Sarandon). Further complicating things, daughter Lyla (Katherine Heigl) disdains her father, which is clear by the way she calls him by his first name, spitting out the single syllable like a bad grape.

And that isn’t all. The groom invited his ultra-Catholic Colombian biological mother and sister to the nuptials. Fearing they may blanch at his adoptive parents’ separation, he (in a “Birdcage”-like twist) requests that Don and Ellie pretend to still be married, just for the weekend.

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Preposterous though it may sound, the story has possibility and might have been well-suited to the same baby boomer ticket buyers who flock to Nancy Meyers’ films (“It’s Complicated,” “Something’s Gotta Give”). But the comedy lacks one very important ingredient: a truly likable, or even mildly relatable, character.

Don takes the title of most offensive persona. The aging lech may be challenging himself to drop as many expletives into each sentence as possible. He brags to his daughter about his talents for “laying pipe” and, in one scene, delivers a two-for-one pickup line, simultaneously hitting on his ex-wife and his paramour. While matriarch Ellie is a bit of a nightmare, especially when discussing tantric sex at the dinner table, daughter Lyla edges her out on the most-horrifying-character list. She is bitter, mean and possibly incapable of smiling, and Heigl’s politician’s-wife hair style hardly softens her image.

Vivacious pug enthusiast Bebe is far more enjoyable, although between her embroidery room, homemade kombucha and organic catering company, she’s more caricature than authentic individual.

When Williams appears onscreen in a priest’s collar, something funny -- meaning interesting, not comedic -- happens. Reflexively, the audience begins to giggle. He doesn’t have to say a word. And maybe it would be better if he didn’t, because even the seasoned comedian can’t land the movie’s lazy jokes, as he questions the betrothed couple about premarital sex and contraception.

Among the few bright spots is Topher Grace, who plays the Griffins’ oldest son, a virgin doctor pushing 30. His timing and facial expressions may remind audiences why the actor once starred in his own sitcom. And Seyfried has an effortless screen presence that, even amid the contrived farce, feels natural.

As the movie trudges toward its conclusion, the script attempts to ferret out emotion from the audience. But by then there’s no chance of feeling sympathy for a cad and his mostly mean-spirited family. Even the susceptible softies, who always cry at weddings, would leave this theater dry-eyed, not to mention feeling a little empty inside.